A current challenge for optical communications is to expand the capacities of optical backbone network to cope with the possible future explosive expansion of information communications traffic. Various approaches are being taken to the challenge. One of the approaches is to carry out research and development regarding an improvement in usage efficiency of an optical frequency band.
In optical network systems, optical frequency bands are used in accordance with the Dense Wavelength Division Multiplexing (DWDM) system standardized by the Telecommunication Standardization sector of the International Telecommunication Union (ITU-T). In the DWDM system, the entire available optical frequency band is divided into narrow segments by a grid with constant width, called a wavelength grid, and optical signals in one wavelength channel are allocated within a grid spacing (ITU-T recommendation G.694.1).
In a flexible frequency grid that is standardized by the ITU-T recommendation G.694.1, the minimum channel spacing is set at 12.5 GHz instead of 50 GHz used conventionally, and an optical frequency slot width is variable by 12.5 GHz. This makes it possible to allocate a frequency slot of different widths to each optical path; accordingly, it becomes possible to minimize an optical bandwidth to be allocated to an optical path. In other words, it is possible to reduce unnecessary band allocation by allocating only a needed optical frequency band, which makes it possible to improve the usage efficiency of the optical frequency band.
Specifically, an optical network management device constituting an optical network system calculates the minimum number of optical frequency slots to be allocated to an optical path, and a center optical frequency of an optical path, depending on a traffic demand. In this case, a lot of optical frequency slots are required because the S/N ratio (signal to noise ratio) of an optical signal deteriorates as the optical path length becomes longer even though optical signals have the same signal speed.
A variable band optical transmitter and receiver device constituting an optical network system changes an optical frequency bandwidth of an optical signal to be inputted and output based on the calculation results by the optical network management device. In this example, it is possible to change the optical frequency bandwidth by changing an optical modulation method (see Patent Literature 1, for example).
Specifically, if signal speeds are the same, the optical frequency bandwidth of an output optical signal becomes larger for an optical modulation method in which the number of modulation symbols is smaller. This is because the optical frequency usage efficiency becomes higher as the number of levels of multilevel modulation becomes larger. For instance, the optical frequency bandwidth of the optical signal generated by a QPSK (Quadrature Phase Shift Keying) optical modulation method is larger than the optical frequency bandwidth of the optical signal generated by a 16QAM (Quadrature Amplitude Modulation) optical modulation method. The optical frequency bandwidth of the optical signal generated by a BPSK (Binary Phase Shift Keying) optical modulation method is larger than the optical frequency bandwidth of the optical signal generated by the QPSK optical modulation method. That is to say, the optical frequency bandwidth of the optical signal to be generated becomes larger in order of 16QAM<QPSK<BPSK.
On the other hand, a transmission distance, that is, the length of an optical path that can be set becomes longer for an optical modulation method in which the number of modulation symbols is smaller. The reason is that the distance between symbols becomes longer as the number of modulation symbols becomes smaller; consequently, the optical S/N ratio after having been transmitted over a predetermined distance becomes larger.
Accordingly, it is possible to allocate an optical frequency slot to an optical path based on the result calculated by the optical network management device by selecting an optical modulation method in an optical transmitter and receiver device depending on a transmission distance of an optical signal, that is, an optical path length.